Port smessage to RocksDB, make RocksDB a hard dep

The secure-messaging store (smsgDB) used the LevelDB API directly. Mass-
mapped to the equivalent RocksDB types: leveldb::DB/Status/WriteBatch/
Iterator/Slice/ReadOptions/WriteOptions/WriteBatch::Handler -> rocksdb::*.
The RocksDB API surface for our usage is binary-compatible — pure namespace
substitution, no semantic changes. Consumers in rpcsmessage.cpp and
qt/messagemodel.cpp updated to match.

RocksDB now becomes a hard build dependency (was optional behind
BUILD_ROCKSDB). The chain-DB rocksdb backend is consequently always
available; -chaindb=leveldb remains the default until the Phase-4
LevelDB retirement. Removed the BUILD_ROCKSDB cmake option, the
#ifdef BUILD_ROCKSDB guards in txdb*, and the runtime error path
that triggered when the flag was off.

CI updated: librocksdb-dev (Ubuntu), mingw-w64-x86_64-rocksdb (MSYS2),
and rocksdb (Homebrew) added to all build jobs.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

.github/workflows/build-all.yml

@@ -20,7 +20,7 @@ jobs:
           sudo apt-get update
           sudo apt-get install -y build-essential cmake ninja-build \
             libboost-all-dev libssl-dev libdb++-dev libleveldb-dev \
-            libevent-dev libminiupnpc-dev zlib1g-dev
+            librocksdb-dev libevent-dev libminiupnpc-dev zlib1g-dev
 
       - name: Configure
         run: |
@@ -60,7 +60,7 @@ jobs:
           sudo apt-get update
           sudo apt-get install -y build-essential cmake ninja-build \
             libboost-all-dev libssl-dev libdb++-dev libleveldb-dev \
-            libevent-dev libminiupnpc-dev zlib1g-dev
+            librocksdb-dev libevent-dev libminiupnpc-dev zlib1g-dev
 
       - name: Configure with sanitizers
         run: |
@@ -105,6 +105,7 @@ jobs:
             mingw-w64-x86_64-libevent
             mingw-w64-x86_64-miniupnpc
             mingw-w64-x86_64-zlib
+            mingw-w64-x86_64-rocksdb
 
       - name: Set VERSION
         run: |
@@ -253,6 +254,7 @@ jobs:
             mingw-w64-x86_64-libevent
             mingw-w64-x86_64-miniupnpc
             mingw-w64-x86_64-zlib
+            mingw-w64-x86_64-rocksdb
 
       - name: Configure
         run: |
@@ -318,7 +320,7 @@ jobs:
           sudo apt-get install -y build-essential cmake ninja-build \
             qtbase5-dev qttools5-dev-tools \
             libboost-all-dev libssl-dev libdb++-dev \
-            libleveldb-dev libevent-dev libminiupnpc-dev zlib1g-dev
+            libleveldb-dev librocksdb-dev libevent-dev libminiupnpc-dev zlib1g-dev
 
       - name: Configure
         run: |
@@ -434,7 +436,7 @@ jobs:
           sudo apt-get update
           sudo apt-get install -y build-essential cmake ninja-build \
             libboost-all-dev libssl-dev libdb++-dev libleveldb-dev \
-            libevent-dev libminiupnpc-dev zlib1g-dev
+            librocksdb-dev libevent-dev libminiupnpc-dev zlib1g-dev
 
       - name: Configure
         run: |
@@ -561,7 +563,7 @@ jobs:
 
       - name: Install dependencies
         run: |
-          brew install cmake ninja qt@5 openssl@3 boost berkeley-db@5 leveldb libevent miniupnpc
+          brew install cmake ninja qt@5 openssl@3 boost berkeley-db@5 leveldb rocksdb libevent miniupnpc
 
       - name: Configure
         run: |